EP1211535A2 - Procédé et système d'alignement d'un train optique à filtre accordable - Google Patents

Procédé et système d'alignement d'un train optique à filtre accordable Download PDF

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Publication number
EP1211535A2
EP1211535A2 EP01121711A EP01121711A EP1211535A2 EP 1211535 A2 EP1211535 A2 EP 1211535A2 EP 01121711 A EP01121711 A EP 01121711A EP 01121711 A EP01121711 A EP 01121711A EP 1211535 A2 EP1211535 A2 EP 1211535A2
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EP
European Patent Office
Prior art keywords
optical
optical signal
tunable filter
lens
alignment
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP01121711A
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German (de)
English (en)
Other versions
EP1211535A3 (fr
Inventor
Jeffrey A. Korn
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Axsun Technologies LLC
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Axsun Technologies LLC
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Filing date
Publication date
Application filed by Axsun Technologies LLC filed Critical Axsun Technologies LLC
Publication of EP1211535A2 publication Critical patent/EP1211535A2/fr
Publication of EP1211535A3 publication Critical patent/EP1211535A3/fr
Withdrawn legal-status Critical Current

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    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/42—Coupling light guides with opto-electronic elements
    • G02B6/4201—Packages, e.g. shape, construction, internal or external details
    • G02B6/4246—Bidirectionally operating package structures
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29346—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by wave or beam interference
    • G02B6/29358—Multiple beam interferometer external to a light guide, e.g. Fabry-Pérot, etalon, VIPA plate, OTDL plate, continuous interferometer, parallel plate resonator
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29346—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by wave or beam interference
    • G02B6/29361—Interference filters, e.g. multilayer coatings, thin film filters, dichroic splitters or mirrors based on multilayers, WDM filters
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29379—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device
    • G02B6/29395—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device configurable, e.g. tunable or reconfigurable
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/32—Optical coupling means having lens focusing means positioned between opposed fibre ends

Definitions

  • Tunable microelectromechanical (MEMS) filters are playing an increasingly important role in modern optical communication networks. They are being deployed in applications ranging from systems that monitor the spectral characteristics of the optical signals that are transmitted through the networks to tuning elements for receivers and laser devices, for example. These applications are most important in modern wavelength division multiplex (WDM) systems in which many optical carrier signals are combined into a common fiber at different carrier wavelengths. Still further applications include matched-noise filtering at receivers, and add-drop devices.
  • WDM wavelength division multiplex
  • Still further applications include matched-noise filtering at receivers, and add-drop devices.
  • One common tunable filter configuration uses two nominally parallel mirrors, in which at least one of the mirrors is translated relative to the other mirror to achieve the tuning function.
  • Such tunable filters are often referred to as tunable Fabry-Perot filters.
  • at least one of the two mirrors is curved to ease assembly tolerances.
  • Some conventional systems integrate the tunable filter into the larger optical system by simply locating it between two fiber pigtails; one fiber pigtail emits the optical signal to be filtered and the other fiber pigtail collects the filtered optical signal after its transmission through the tunable filter.
  • the tunable filter is oriented to be orthogonal to the axis extending between the fiber endfaces.
  • Improper or imprecise alignment can impact dynamic range of the optical train and excite higher order modes in the optical filter. These higher order modes are undesirable because they can cause confusion as to how many WDM channels exist in an optical signal, for example. It can also cause undesirable inter-channel crosstalk.
  • the invention features an alignment process for a fiber optic system including at least one lens and a tunable filter element.
  • the process comprises transmitting an optical signal into the system and detecting the signal.
  • the positions of the elements are then manipulated relative to a bench for the system.
  • the train is aligned to maximize a ratio between a lower order mode and a next higher order mode.
  • the invention features a fiber optic alignment system for an optical train comprising at least a lens and a tunable filter.
  • the system comprises an optical signal source, an optical signal detector for detecting the optical signal after transmission through as least part of the optical train, and a manipulation system for moving the lens and tunable filter element relative to each other in response to the optical signal detector.
  • Fig. 1 shows an exemplary optical system to which the present invention is applicable.
  • the present invention is most applicable to fiber-optic-based optical systems where the single-transverse mode optical signal is provided to the optical system via a fiber 110.
  • the optical signal that is emitted from the fiber's endface 112 forms a diverging beam, which is typically collimated or has its collimation improved to form a beam waist by a lens, such as a first convex lens 114.
  • the invention is extendable to the more complex system where the optical signal beam is then intended to be focused by a second lens 116 into the cavity 118 of a tunable filter 120.
  • the present invention is applicable to small optical systems and beam sizes.
  • the single mode fiber has a mode size of approximately 10 microns.
  • the maximum beam size in the trains of Figs. 1, 9, and 10 is less than 400 microns and propagates between 0.5 and 2.0 millimeters above a bench 135 to which the optical components are attached, see Fig. 10. In the preferred embodiment, the maximum beam size is less 150 microns or about 100 microns.
  • the lenses 114, 116 are fabricated using the mass transport technique described in U.S. Pat. No. 5,618,474.
  • the diameter of the lenses is between 100 and 500 microns. Typically the diameters are between 150 and 300 microns.
  • the invention is compatible with other types of microlenses such as those generated by binary optics, gradient index processes, or refractive element replication, for example.
  • the filter is a MEMS device as described in Patent Application Serial No. 09/649,168, filed on August 25, 2000, by Flanders, et al ., entitled Tunable Fabry-Perot Filter.
  • the illustrated tunable filter 120 comprises a first, typically partially, reflecting mirror 122 and a second, typically partially, reflecting mirror 124.
  • the optical length of the optical cavity is tunable as illustrated by arrow 126.
  • the second mirror is curved as is common to reduce alignment tolerances relative to other mirrors that define the optical cavity 118.
  • an alignment jig system is connected to the optical system.
  • the jig system comprises a circulator or coupler 132 connected to the distal end of the fiber 110.
  • the circulator allows light from an optical signal generator 134, preferably a single frequency signal from a source, such as a distributed-feedback semiconductor laser.
  • the circulator 132 directs back-reflected signals returning from the optical system 100 to a photodetector 136.
  • Fig. 2 illustrates the transfer function of the optical system illustrated in Fig. 1.
  • the schematically illustrated misalignment between the optical components leads to the excitation of higher order modes in the optical system 100.
  • a transmission peak of a higher-order mode is also present at approximately 1536 nm. This higher-order mode is undesirable and should be ideally removed from or suppressed in the system.
  • Fig. 3 illustrates a first step of the alignment process in which a mirror 138 or other reflective element of the alignment jig system is inserted into the optical train of the optical system 100.
  • the mirror 138 is inserted between lens #1 114 and lens #2 116.
  • the mirror 138 is inserted into the optical train such that it is orthogonal to the nominal optical axis 130.
  • the optical signal generator 134 is activated and the backward coupling efficiency of the optical system 100 with the inserted mirror 138 is detected by detecting the level of optical signal received by the photodetector 136. Note that in this step, the single frequency source or a broader band source could be used.
  • the position of the fiber 110 and lens #1 114 are moved relative to each other by an optical component manipulation system so that the backward coupling efficiency is maximized. Specifically, the fiber 110 and lens 114 are moved relative to each other such that the optical signal detected by the photodetector 136 is maximized. Once aligned, these two components define the nominal optical axis 130 for the subsequent alignment process and the aligned system.
  • the positions of the fiber and the lens are manipulated in two dimensions in a plane that is orthogonal to the optical axis by the manipulation system. Specifically, their positions are manipulated in the y-axis direction illustrated by arrows 150 and 152, but also in the x-axis direction, which extends into the page in the illustration of Fig. 3, for example.
  • Fig. 4 illustrates the next step in the alignment process. Specifically, once the alignment of the fiber 110 and lens #1 114 has been achieved within the desired tolerances, the mirror 138 is removed as illustrated in Fig. 4. In this configuration, the optical signal generator 134 is again activated. Its signal is selected relative to the transfer function of the tunable filter 120 such that the wavelength is not within the passband of the tunable filter 120, i.e ., the optical signal is off-resonant light relative to the tunable filter 120. The light, however, is within a stop band of the tunable filter 120. Specifically, the light wavelength is confined to at least the tuning wavelength of the tunable filter 120.
  • the optical signal emitted from the fiber 110 into the optical system 100 is reflected by the tunable filter 120.
  • the position of lens #2 is translated in a plane that is orthogonal to the nominal optical axis, i.e ., the x, y plane, see arrow 156, by the manipulation system, to again maximize the back-reflection detected by the detector 136.
  • Fig. 5 illustrates the aligned optical system 100 after the manipulation of lens #2 116 is performed to maximize the back reflection efficiency of the optical signal 100 at the off-resonant wavelength.
  • Fig. 6 is a flow diagram summarizing the above-described alignment process and optical component manipulation by the manipulation system.
  • the mirror 138 is inserted into the optical train of the optical system 100 between lens #1 114 and lens #2 116.
  • the fiber 110 and lens #1 114 are aligned to maximize backward coupling efficiency.
  • the mirror 138 is removed in step 214.
  • lens #2, and also possibly the filter 120 is translated while exciting the system with off-resonant light to again maximize the back-reflection, now from the filter 120.
  • Fig. 7 illustrates another configuration of an optical system to which the present invention is applicable.
  • the configuration of the optical filter is essentially reversed from that illustrated in Fig. 1, for example. Specifically, as illustrated in Fig. 7, the optical signal is initially received and transmitted through a curved reflecting element 142, and then reflected off of a flat or curved reflecting element 144 to thereby form the Fabry-Perot filter cavity 118.
  • an optical coating 140 is added to the flat substrate of the curved reflecting element 144 of the tunable filter. It is coated to be reflective at a predetermined wavelength within the transmission spectrum of the fiber tunable filter system 100, or at least lenses 114, 116 and fiber 110.
  • the optical signal generator is then tuned to emit at this predetermined wavelength during the final alignment step of lens #2 116.
  • a back-reflection is generated to allow the alignment of lens #2 116 by reference to the backward coupling efficiency as described previously in Fig. 4.
  • Fig. 8 illustrates an alignment process for an optical system as illustrated in Fig. 1 according to another embodiment of the present invention.
  • a camera 133 such a CCD array is inserted into the train of the optical system 100 at the position of fiber 110.
  • the camera is inserted such that it is axial with the intended position of the fiber's endface 112.
  • the camera is inserted into the optical train of the optical system prior to the installation of the fiber 110. This configuration is illustrated in Fig. 9.
  • the optical train is excited with an optical signal, which is preferably a single frequency such as generated by a distributed feedback laser, for example.
  • This alignment signal is preferably launched in a direction reverse to the direction of optical signal propagation during normal operation of the optical system.
  • the wavelength of the laser and the tunable filter are mutually adjusted so that the laser emission overlaps with the lowest order mode (TEM 00 ).
  • lens #2, 116 While excited, the surface of lens #2, 116 is imaged onto the camera in step 254. Lens #2 116 is then translated with the manipulation system such that the lens image is coincident with the nominal optical axis 130. This translation occurs in a plane that is orthogonal to the nominal optical axis or the x, y plane defined in Fig. 1.
  • lens #2 116 aligned, the image of the surface of lens #1 is then imaged onto the camera in step 256.
  • Lens #1 114 is then translated by the manipulation system in the x,y, plane in step 258 so that it is similarly coincident and aligned relative to the optical axis 130. Specifically, the center of lens 114 is aligned such that it coincides with the nominal optical axis 130.
  • step 260 the camera is removed and the fiber is attached to the optical system in step 262, typically as part of a final manufacturing step of the optical system 100.
  • the optical system is excited with a broadband source.
  • the optical signal generator 130 is selected to be a broadband source such as a super luminescent light emitting diode SLED.
  • the ratio between the lowest order mode and any higher order mode is then maximized in step 226.
  • the ratio of the lowest order mode in the next higher order mode is maximized in step 226 by positioning the fiber 110 in the x, y plane.
  • Fig. 10 shows the complete optical system of the optical channel monitoring system utilizing the optical filter train discuss with reference to Figs. 1-9.
  • the fiber 110 terminates above an optical bench 135. During normal operation, the optical signal 14 is emitted out of the typically cleaved or cleaved-polished endface of the fiber.
  • the optical signal is typically diverging as it is emitted from the fiber's core. It is collimated by the lens #1 114.
  • a dichroic mirror 140 is used to add a reference signal 111 to the optical signal 14. These dichroic mirrors or filters are typically referred to as WDM filters. In the illustrated implementation, the WDM filter 140 is reflective in a band surrounding 1300 nm, but transmissive in a band surrounding 1500 nm.
  • the 1300 nm reference signal is generated by a light emitting diode 142.
  • the light emitting diode is a super luminescent light emitting diode (SLED).
  • the diverging beam from the LED is collimated by a second collimating lens 144.
  • An etalon 146 is used to convert the relatively wide-band signal from the SLED into a reference signal with stable spectral characteristics. More specifically, the etalon 146 functions as a Fabry-Perot filter with a 200 GigaHertz (GHz) free spectral range (FSR). This effectively converts the SLED's continuous, broadband spectrum into a signal with energy peaks every 200 GHz. These peaks are stable, particularly when the temperature of the system is controlled by a thermoelectric cooler or is otherwise stabilized.
  • GHz GigaHertz
  • a fold mirror 145 redirects the reference signal to the WDM filter 140. It should be noted, however, that this mirror is not required, but is simply used to facilitate integration of the system on a compact bench.
  • the combined optical signal 14/111 is transmitted through an isolator 138. This component is used to prevent back-reflections from the subsequent optical components into the fiber 132.
  • Lens #2 116 is used to focus the collimated combined beam 14/111 onto the tunable filter 120. After the tunable filter, the beam is recollimated by a third collimating lens 152, and transmitted to a second dichroic/WDM filter 154.
  • the second WDM filter 154 functions to separate the filtered reference signal from the filtered optical signal in the filtered beam 16 from the tunable filter 120.
  • the second WDM filter 154 is reflective in a band around 1300 nm, but transmissive in a band around 1500 nm.
  • the filtered reference signal is directed to the wavelength reference detector 122 for optical-electrical conversion.
  • the filtered optical signal is transmitted to the signal detector system 184.
  • the L- and C-bands are separated from each other by a third WDM filter 156.
  • This WDM filter 156 is reflective to the C-band and transmissive to the L-band.
  • the C-band of the WDM signal is detected by a C-band photodiode 158; the L-band is transmitted through the WDM filter 156 to be detected independently by an L-band photodiode 160.
  • more than two bands, such as three or four, are detected simultaneously by adding additional WDM filters and detectors.
  • This system provides for out-of-band calibration. This yields the advantage that the calibration can occur simultaneously with wavelength monitoring. Specifically, one or more of the filter's modes are used for signal detection while another mode is used to simultaneously filter the calibration signal.
  • a similar stable source is used for in-band calibration.
  • One downside to such embodiments is the fact that complex post processing and/or time multiplexing functionality is required upstream of the detectors to switch between signal monitoring and signal calibration.
  • LED sources are used, such as LED sources operating at approximately 1400 nm, such as an InGaAsP SLED.
  • the salient features of the tunable filter 120 are its selectable free spectral range.
  • the free spectral range is 20 nm ⁇ FSR ⁇ 170 nm at 1550 nm wavelength. It preferably also has high finesse, i.e ., greater than 3,000, and a compact size.
  • the filter is as described in Patent Application Serial No. 09/649,168, by Flanders, et al ., entitled Tunable Fabry-Perot Filter, filed on August 25, 2000.
  • a 40 nm FSR is selected. This enables simultaneous scans of the C and L-bands, in addition to calibration relative to the reference band.
  • the FSR of the filter must be greater than the bandwidth of at least one of the bands of interest so that successive modes of the filter can access both bands simultaneously.
  • the FSR must be less than the combined bandwidth of the bands, again to enable simultaneous access.
  • the free spectral range of the tunable filter is increased to 57.5 nm to enable monitoring of the optical service channels that flank the C-and L-bands.
  • a spatial mode aperture is used in conjunction with the tunable filter. Such intra-filter apertures are desirable when extra cavity mode control devices are not used. For example, in some other implementations, a length of single mode fiber follows the filter to attenuate higher order modes.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Optical Couplings Of Light Guides (AREA)
EP01121711A 2000-09-21 2001-09-17 Procédé et système d'alignement d'un train optique à filtre accordable Withdrawn EP1211535A3 (fr)

Applications Claiming Priority (2)

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US666194 2000-09-21
US09/666,194 US6813420B1 (en) 2000-09-21 2000-09-21 Process and system for tunable filter optical train alignment

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Cited By (2)

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WO2002052316A2 (fr) * 2000-12-22 2002-07-04 Axsun Technologies, Inc. Systeme et procede de suppression des modes lateraux par alignement de la trajectoire du filtre a accord variable dans un systeme de fibres optiques
WO2009096927A1 (fr) * 2008-01-31 2009-08-06 Hewlett-Packard Development Company, L.P. Interconnexion optique en espace libre

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US7376169B2 (en) * 2005-03-07 2008-05-20 Joseph Reid Henrichs Optical phase conjugation laser diode
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US11606147B1 (en) * 2022-06-06 2023-03-14 Bae Systems Information And Electronic Systems Integration Inc. Frequency and bandwidth agile optical bench

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WO2002052316A2 (fr) * 2000-12-22 2002-07-04 Axsun Technologies, Inc. Systeme et procede de suppression des modes lateraux par alignement de la trajectoire du filtre a accord variable dans un systeme de fibres optiques
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